Semi-wide angle objective lens

ABSTRACT

A semi-wide angle objective lens in accordance with the present invention comprises five groups of six lens elements including a front group and a rear group in which the front group has a first group which includes a single positive meniscus lens L 1  with its convex surface directed toward the object side, a second group which includes a single negative meniscus lens L 2  with its convex surface directed toward the object side, and a third group which includes a single positive meniscus lens L 3  with its convex surface directed toward the object side; and in which the rear group has a fourth group which includes both a negative lens L 4  with its front concave surface having a large curvature directed toward the object side and a positive lens L 5  with its rear convex surface having a large curvature directed toward an image side with the fourth group lenses cemented together or arranged with an extremely small air space therebetween, and a fifth group which includes a positive lens L 6  characterized in that each of the following conditions is fulfilled: 
     
         1.4 f&lt;|f.sub.2 | 2.0 f                   (1) 
    
     
         0.23 f&lt;r.sub.4 &lt;0.33 f                                     (2) 
    
     
         0.03 f&lt;d.sub.4 &lt;0.08 f                                     (3) 
    
     
         0.8&lt;r.sub.4 /r.sub.5 &lt;1.0                                  (4) 
    
     where, 
     f is the composite focal length of the entire lens system, 
     f 2  is the focal length of the lens L 2  of the second group (f 2  &lt;0), 
     r 4  is the radius of curvature of the rear surface of the lens L 2  of the second group, 
     d 4  is the axial air space between the lens L 2  of the second group and the lens L 3  of the third group, and 
     r 5  is the radius of curvature of the front surface of the lens L 3  of the third group.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to an improved semiwide angle objective lens foruse with single lens reflex cameras.

(2) Description of the Prior Art

In recent years, there has been an increasing trend towards compact sizesingle lens reflex cameras and, along with this trend a requirement hasdeveloped for more compact and wider angle interchangeable lens. Theshortest focal length of a standard lens for a 135 mm film format singlelens reflex camera has been commonly chosen at 50 mm. However, lensshutter cameras have lenses with approximately equal to 40 mm are verypopular. Thus, for SLR cameras, a semi-wide angle objective lens havinga value of about f=40 mm has become a standard lens.

A desirable semi-wide angle objective lens having a compact size wouldhave a preferred value of f=40 mm, an F number of F 2 to F 1.8 and asimple lens configuration of six lens elements or so. It has beenextremely difficult to apply prior art lenses (e.g., Gaussian type sixelement lenses) for this purpose and keep the lens back focal lengthlarge enough for a single lens reflex camera and also provide a fieldangle up to about 56°.

The reason why it was so difficult is that keeping the back focal lengthf_(B) long requires a smaller setting of the positive power of the frontgroup, resulting in an increased positive power of the rear group makingit very difficult to correct residual aberrations with the positivelenses of the rear group.

Another reason for the above mentioned difficulty is that therequirement for a large aperture ranging from F 2 to F 1.8 results in adecreased power of refraction at each lens surface, which leads todifficulty correcting the curvature of the sagittal image surface,making it impossible to maintain a wide angle of view.

Further, even if an attempt is made to obtain a semi-wide angleobjective lens by utilizing a lens system of a so-called retrofocus typee.g. a Gauss type or modified Tessar type in which a negative lens isadded to the front lens element, the total length of the lens system isincreased, or the front lens element is increased in size and the lensis no longer compact.

Because of the reasons mentioned above, it has been extremely difficultto provide a six lens element semi-wide angle objective lens of simpleconfiguration having a large aperture and a compact size.

BRIEF SUMMARY OF THE INVENTION

It is an overall object of the present invention to provide a new anduseful semi-wide angle objective lens for use with single lens reflexcameras.

It is further principal object of the present invention to provide alens system for an extremely compact semi-wide angle objective lenshaving a simple configuration of five groups of six lens elements inwhich the back focal length f_(B) has a value of 0.85 times of thecomposite focal length f, an angle of view of 56°, an aperture rangefrom F 2 to F 1.8 and in which all aberrations are well corrected.

The above and other objects of the present invention will be apparentfrom the following detailed description of a preferred embodiment of thepresent invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above description as well as further objects, features, andadvantages of the present invention will be more fully understood byreference to the following description of a presently preferred butnonethless illustrative embodiment in accordance with the presentinvention, when taken in connection with the accompanying drawing,wherein

FIGS. 1A and 1B show a basic structural view of a semi-wide angle lensin accordance with the present invention with FIG. 1A showing anarrangement in which two lens elements in the fourth group are cementedtogether and with FIG. 1B showing an arrangement in which two lenselements in a similar fourth group are arranged with an extremely smallair space being left therebetween.

FIGS. 2A, 2B, 2C, and 2D show, as labeled, the coma, sphericalaberration, astigmatism, and distortion curves for a first embodimenthaving the basic construction shown in FIG. 1A;

FIGS. 3A, 3B, 3C, and 3D show, as labeled, the coma, sphericalaberration, astigmatism, and distortion curves for a second embodimenthaving the basic construction shown in FIG. 1A;

FIGS. 4A, 4B, 4C, and 4D show, as labeled, the coma, sphericalaberration, astigmatism, and distortion aberration curves for a thirdembodiment having the basic construction shown in FIG. 1A.

In each of these figures, the solid line in the spherical aberrationcurve shows the spherical aberration when d=line (a wave length: 587.56mμ) is applied and the dotted line shows the spherical aberration wheng-line (a wave length: 435.84 mμ) is applied. The solid line in theastigmatism shows a sagittal and the dotted line shows a meridional.

FIGS. 5A, 5B, 5C and 5D show, as labeled, the coma, sphericalaberration, astigmatism, and distortion curves for a fourth embodimenthaving the basic construction shown in FIG. 1B; and

FIGS. 6 6B, 6C, and 6D show, as labeled, the coma spherical aberration,astigmatism, and distortion curves for a fifth embodiment in accordancewith the basic construction shown in FIG. 1B.

In FIG. 1A,

L₁, L₂, L₃, . . . L₆ : designates the lenses counted in sequence fromthe object side

r₁, r₂, r₃, . . . r₁₁ : designates the radius of curvature of the lenssurfaces measured in sequence from the object side,

d₁, d₃, d₅, d₇, d₈, d₁₀ : designates the axial thickness of the lensesmeasured in sequence from the object side;

d₂, d₄, d₆, d₉ : designates the axial air space between the lensesmeasured in sequence from the object side;

and in FIG. 1(B),

L₁, L₂, L₃, . . . L₆ : designates the lenses counted in sequence fromthe object side

r₁, r₂, r₃, . . . r₁₂ : designates the radius of curvature of the lensessurface measured in sequence from the object side

d₁, d₃, d₅, d₇, d₉, d₁₁ : designates the axial thickness of the lensmeasured in sequence from the object side, and

d₂, d₄, d₆, d₈, d₁₀ : designates the axial air space between the lensesmeasured in sequence from the object side

The solid line in the spherical aberration shows the sphericalaberration when d-line (a wave length: 587.56 mμ) is applied and thedotted line shows the spherical aberration when g-line (a wave length:435.84 mμ) is applied. The solid line in the astigmatism shows asagittal and the dotted line shows a meridional.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As apparent from the new type of lens shown in FIG. 1 A and B in whichthe arrangement of the front group of the Gauss type lens of five groupsof six lens elements is changed by changing the arrangement of apositive lens, a positive lens and a negative lens to an arrangement ofa positive lens, a negative lens and a positive lens, the presentinvention enables the provision of a lens system for an extremelycompact and useful semi-wide angle lens having a back focal length f_(B)of about 0.85 times as that of a composite focal length f, an angle ofview of 56°, and an aperture of F 2 to F 1.8 in which all aberrationsare well corrected. A simple, common lens configuration of five groupsof six lens elements, which has various problems when used as aninterchangeable lens for single lens reflex cameras, may be improvedupon by providing a lens system comprising five groups of six lenselements including a front group a rear group in which a first groupincludes a single positive meniscus lens L₁ having its convex surfacedirected toward a object side, the second group includes a singlenegative meniscus lens L₂ having its convex surface directed toward theobject side, and a third group includes a single positive meniscus lensL₃ with its convex surface having a large curvature directed toward theobject side and in which the rear group includes the fourth group whichincludes both a negative lens L₄ with its front concave surface having alarge curvature directed toward the object side and a positive lens L₅with its rear convex surface having a large curvature being directedtoward the image side with the fourth group lenses cemented together orarranged with an extremely small air space therebetween and a fifthgroup includes a positive lens L₆ in which each of the followingconditions are fulfilled.

    1.4f<|f.sub.2 |<2.0 f                    (1)

where,

f is the composite focal length of the entire lens system,

f₂ is the focal length of the lens L₂ of the second group;

    0.23 f<r.sub.4 <0.33 f                                     (2)

where, r₄ is the radius of curvature of the rear surface of the lens L₂of the second group;

    0.03 f<d.sub.4 <0.08 f                                     (3)

where, d₄ is the axial air space between the lens L₂ of the second groupand the lens L₃ of the third group; and

    0.8<r.sub.4 /r.sub.5 <1.0                                  (4)

where, r₅ is the radius of curvature of the front surface of the lens L₃of the third group

A technical background of the present invention and the meaning ofseveral conditions are described in more detail as follows.

A new type of the lens in accordance with the present invention is madeby replacing the arrangement of prior art Gauss-type lenses, whichinclude five groups of six lens elements of which the front groupincludes a positive lens, a positive lens, and a negative lens, with anarrangement of a positive lens, a negative lens and a positive lens.

As mentioned in the description of the prior art, a semi-wide angleobjective lens with the back focal length long enough for aninterchangeable lens and with an aperture large enough for a standardlens is very difficult to provide with the prior art Gauss type orTessar-type lens configurations.

To avoid the problems mentioned earlier, it is necessary to provide alens which has a suitably large positive power in the front group as theprior art Gauss type lenses making a large aperture possible and whichalso has a suitably large emerging paraxial ray height at the endsurface of the front group the prior art Tessar type lenses making along back focal length possible; the new type of lens of the presentinvention satisfies these properties.

Namely, the suitably large power in the front group is made possible inthe lens of the present invention by using the two positive lenses, oneof which is located in the first position of the lens system, and onenegative lens similar to the prior art Gauss-type lenses; thus keepingthe power of the front group large and comparable to that of theGauss-type lenses.

On the other hand, by changing the arrangement of the lenses in thefront group from that of the Gauss-type lenses, which is a positivelens, a positive lens, and a negative lens, to the arrangement of apositive lens, a negative lens, and a positive lens, the type of lens ofthe present invention enables the attaining of a long back focal length.

Namely, the forward arrangement of the negative lens in the front groupmakes the emerging paraxial ray height at the end surface of the frontgroup comparable to that of a Tessar type lens, and thus makes itpossible for the total lens to have a long back focal length.

In view the arrangement described, major means for realizing a compactlens system having a superior aberration correction is expressed underthe following conditions

    1.4 f<f.sub.2 <2.0 f                                       (1)

    0.23 f<r.sub.4 <0.33 f                                     (2)

    0.03 f<d.sub.4 <0.08 f                                     (3)

    0.8<r.sub.4 /r.sub.3 <1.0                                  (4)

Equation (1) shows a condition in which the negative lens arranged asthe lens L₂ of the second group has a relatively large divergence actionmaking it possible to obtain a required back focal length f_(B) withoutdecreasing the composite power of the lenses of the front group when thesingle focal length is in the range indicated above.

Equation (2) shows a condition in which a substantially large radius ofcurvature r₄ of the lens surface of the meniscus lens L₂ of the secondgroup directed toward the image side is provided, and a center of thecurvature is approached toward the center of the lens in order toprevent a sagittal image surface from being curved, and then a spacebetween the lens L₂ of the second group and the lens L₃ of the thirdgroup is made small as indicated in the equation (3) to keep the sizecompact and also to keep the effect described below of a radius ofcurvature r₅ at the object side of the lens L₃ of the third group inreference to the equation (4). That is, these equations show theconditions to decrease the aberrations generated by the forwardpositioned negative lens, which aberrations are larger than those of theprior art arrangement of a positive lens, a negative lens and a negativelens, and thus provide a fast lens.

The reason for the above mentioned setting of the limitation values ofeach of the conditional equations are described as follows.

Negative power of the lens L₂ of the second group exceeding the lowerlimit of the equation (1) will necessarily increase the positive powerof the lens L₃ of the third group and thus cause generation of anexcessive aberration of each of the surfaces, making it impossible toprovide a fast lens. In turn, negative power of the lens L₂ of thesecond group exceeding the upper limit of the equation (1) will decreasethe diversive refractive power of the lens L₂ of the second group andthus it is impossible to obtain the required back focal length f_(B).

Exceeding the lower limit of the equation (2) will make the negativerefractive power excessive at the rear surface of the lens L₂ of thesecond group, and thus it will become impossible to correct thespherical aberration and coma, and further make it impossible to providea fast lens. Conversely, exceeding the upper limit of the equation (2)will result in the center of curvature of the rear surface of the lensL₂ being moved farther away from the center of the total lens systemdecreasing the effect of correction of the sagittal image surface.

When the axial air space d₄ between the lens L₂ of the second group andthe lens L₃ of the third group is small and exceeds the lower limit ofequation (3), a positive refractive power at the front surface of lensL₃ decreases in reference to the equation (4), and then it becomesimpossible to provide correction of the higher order sphericalaberrations which may be generated in the negative lens L₂. To thecontrary, when the axial air space d₄ exceeds the upper limit, apositive refractive power at the front surface of said lens L₃increases, and thus it causes not only the tendency for the imagesurface to curve towards object side, but also the increase in size ofthe lens.

Equation (4) provides the relationship between the radii of curvature r₄and r₅ with respect to a range of the axial air space d₄ given by theequation (3). When the value of r₄ /r₅ exceeds the lower limit, it willcause an excessive negative refractive power in the front group, and, ina manner similar to the case in which the lower limit of the equation(3) is exceeded, and it is not possible to correct the sphericalaberration and, further, it becomes impossible to provide a fast lens.When the upper limit is exceeded, it tends to increase the curvature ofthe image surface in the same manner as when the upper limit of theequation (3) is exceeded.

That is, under each of the conditions of the equations (1), (2), (3) and(4), it becomes possible to have the front group of the lens systemsatisfying the properties discussed earlier.

Thus, it is not necessary to have a large positive power in the reargroup and thus the simple configuration of the prior art of only twopositive lenses may be provided making it possible to correctaberrations easily while keeping the size of the lens compact andkeeping the back focal length f_(B) sufficiently long enough. Further,in order to effect the correction of the aberration, it is preferable toprovide the following conditions for the lenses of the rear group;

    1.8<n.sub.6                                                (5)

    20<ν.sub.5 -ν.sub.4                                  (6)

where, n₆ is the refractive index of the glass of the lens L₆ of thefifth group with respect to d-line, ν₄ and ν₅ are Abbe numbers given byd-line of the component glass of the lenses L₄ and L₅ of the fourthgroup.

In order to keep the positive power born by the two positive lenses ofthe rear group as large as possible even if a positive power of thefront group is made to be a relatively large value and a positive powerof the rear group is not too high by fulfilling each of the abovedescribed conditions and further to keep a large aperture lens of thepresent invention, it is preferable to maintain the refractive indiciesn of the two positive lenses as large as possible. However, it isrequired that the refractive index of the lens L₄ is large, therefractive index of the lens L₅ small, ν of the lens L₄ small, and therefractive index of the lens L₅ large in order to provide correction forthe sagittal image surface in the fourth group and correction for thechromatic aberration and thus it is preferable to have a refractiveindex of n₆ as large as possible.

When it is less than the limit of the equation (5), it is impossible toprovide proper corrections for the spherical aberration, distortion etc.in the lenses of the rear group. Along with this effect, the conditionshown in the equation (6) is an essential condition of achromatism inthe lenses L₄ and L₅ of the fourth group under such condition as apowerful achromatism in the lens L₆ of the fifth group may not beobtained and thus it becomes impossible to provide a chromaticcorrection in the rear group when it is less than the limitation.

Practical embodiments of the present invention will now be describedherebelow.

EMBODIMENT 1

    ______________________________________                                        f = 1 F 1.8                                                                             2ω = 56°                                                                     f.sub.B = 0.8464                                                                         Σd = 0.7053                            ______________________________________                                        r.sub.1 = 0.9951                                                                        d.sub.1 = 0.0725                                                                          n.sub.1 = 1.71300                                                                        ν.sub.1 = 53.9                            r.sub.2 = 3.9769                                                                        d.sub.2 = 0.0024                                                    r.sub.3 = 0.4274                                                                        d.sub.3 = 0.0483                                                                          n.sub.2 = 1.58144                                                                        ν.sub.2 = 40.7                            r.sub.4 = 0.2873                                                                        d.sub.4 = 0.0531                                                    r.sub.5 = 0.3052                                                                        d.sub.5 = 0.0966                                                                          n.sub.3 = 1.67003                                                                        ν.sub.3 = 47.3                            r.sub.6 = 0.3126                                                                        d.sub.6 = 0.2029                                                    r.sub.7 = -0.3341                                                                       d.sub.7 = 0.0217                                                                          n.sub.4 = 1.75520                                                                        ν.sub.4 = 27.5                            r.sub.8 = 38.9089                                                                       d.sub.8 = 0.1329                                                                          n.sub.5 = 1.67790                                                                        ν.sub.5 = 55.3                            r.sub.9 = d.sub.9 = 0.0024                                                    r.sub.10 = 11.2319                                                                      d.sub.10 = 0.0725                                                                         n.sub.6 = 1.88300                                                                        ν.sub.6 = 40.8                            r.sub.11 = -0.9167                                                            ______________________________________                                         f.sub.2 = -1.725                                                         

This is an embodiment of FIG. 1A, and where,

r₁, r₂, . . . r₁₁ designates the radius of curvature of the lensesmeasured in sequence from the object side,

d₁, d₃, d₅, d₇, d₈, d₁₀ designates the axial thickness of the lensesmeasured in sequence from the object side,

d₂, d₄, d₆, d₉ designates the axial air spaces between the lensesmeasured in sequence from the object side,

n₁, n₂, . . . n₆ designates the refractive index of the optical glass ofthe lenses with respect to the d-line measured in sequence from theobject side, and

ν₁, ν₂, . . . designates the Abbe number of the optical glass of thelenses measured with respect to d-line in sequence from the object side.

    ______________________________________                                        Seidel's aberrations of Embodiment 1.                                         S         C         A         P       D                                       ______________________________________                                        1   0.2466    0.1487    0.0896  0.4183  0.3061                                2   0.0259    -0.0982   0.3732  -0.1047 -1.0202                               3   0.5778    0.1463    0.0371  0.8602  0.2273                                4   -6.6261   -0.2376   -0.0085 -1.2799 -0.0462                               5   5.6800    0.6946    0.0849  1.3146  0.1712                                6   -1.6196   -0.6435   -0.2557 -1.2836 -0.6116                               7   -3.4114   1.2462    -0.4553 -1.2878 0.6368                                8   -0.0586   -0.0691   -0.0814 -0.0007 -0.0967                               9   2.6149    -0.4894   0.0916  1.0187  -0.2078                               10  -0.0001   0.0027    -0.0750 0.0418  0.9239                                11  2.9679    -0.6796   0.1556  0.5116  -0.1528                               ______________________________________                                        Σ                                                                           0.3972    0.0211    -0.0438 0.2085  0.1300                                ______________________________________                                    

Seidel's aberrations show the value when the distance of the object sideis set to an infinte point and a diaphragm is set to a position of 0.101rearward of the sixth lens surface, and the symbols in the table are asfollows.

S: spherical aberration

C: coma

A: astigmatism

P: Petzval's condition

D: distortion

Σ: sum of each of the aberrations

EMBODIMENT 2

    ______________________________________                                        f = 1 F 1.84                                                                            2ω = 56°                                                                     f.sub.B = 0.8443                                                                         Σd = 0.6940                            ______________________________________                                        r.sub.1 = 0.9963                                                                        d.sub.1 = 0.0724                                                                          n.sub.1 = 1.71300                                                                        ν.sub.1 = 53.9                            r.sub.2 = 3.9165                                                                        d.sub.2 = 0.0024                                                    r.sub.3 = 0.4165                                                                        d.sub.3 = 0.0458                                                                          n.sub.2 = 1.58144                                                                        ν.sub.2 = 40.7                            r.sub.4 = 0.2851                                                                        d.sub.4 = 0.0531                                                    r.sub.5 = 0.3015                                                                        d.sub.5 = 0.0965                                                                          n.sub.3 = 1.67003                                                                        ν.sub.3 = 47.3                            r.sub.6 = 0.3057                                                                        d.sub.6 = 0.1978                                                    r.sub.7 = -0.3287                                                                       d.sub.7 = 0.0210                                                                          n.sub.4 = 1.76182                                                                        ν.sub.4 = 26.6                            r.sub.8 = 8.4698                                                                        d.sub.8 = 0.1303                                                                          n.sub.5 = 1.71300                                                                        ν.sub.5 = 53.9                            r.sub.9 = d.sub.9 = 0.0024                                                    r.sub.10 = 11.3076                                                                      d.sub.10 = 0.0724                                                                         n.sub.6 = 1.83400                                                                        ν.sub.6 = 37.2                            r.sub.11 = -0.8810                                                            ______________________________________                                         f.sub.2 = -1.782                                                         

This is another embodiment of FIG. 1(A). Each of the symbols used hereinis the same as that used in Embodiment 1.

    ______________________________________                                        Seidel's aberrations of Embodiment 2.                                         S         C         A         P       D                                       ______________________________________                                        1   0.2457    0.1500    0.0915  0.4178  0.3109                                2   0.0243    -0.0944   0.3662  -0.1063 -1.0085                               3   0.6707    0.1598    0.0381  0.8828  0.2194                                4   -6.7190   -0.2515   -0.0094 -1.2897 -0.0486                               5   5.8131    0.7110    0.0870  1.3307  0.1734                                6   -1.7360   -0.6758   -0.2631 -1.3124 -0.6133                               7   -3.5823   1.2938    -0.4672 -1.3155 0.6438                                8   -0.0477   -0.0511   -0.0548 -0.0019 -0.0608                               9   2.6768    -0.5298   0.1049  1.0412  -0.2268                               10  -0.0003   0.0048    -0.0886 0.0402  0.8934                                11  3.0449    -0.6941   0.1582  0.5162  -0.1537                               ______________________________________                                        Σ                                                                           0.3902    0.0226    -0.0373 0.2031  0.1291                                ______________________________________                                    

Seidel's aberrations show the value when the distance of the object sideis set to an infinite point and a diaphragm is set to a position of0.099 rearward of the sixth lens surface, and the symbols in the tableare as follows.

S: spherical aberration

C: coma

A: astigmatism

P: Petzval's condition

D: distortion

Σ: sum of each of the aberrations

EMBODIMENT 3

    ______________________________________                                        f = 1 F 2 2ω = 56°                                                                     f.sub.B = 0.8518                                                                         Σd = 0.6359                            ______________________________________                                        r.sub.1 = 0.9272                                                                        d.sub.1 = 0.0689                                                                          n.sub.1 = 1.71300                                                                        ν.sub.1 = 53.9                            r.sub.2 = 3.3721                                                                        d.sub.2 = 0.0024                                                    r.sub.3 = 0.4119                                                                        d.sub.3 = 0.0447                                                                          n.sub.2 = 1.58144                                                                        ν.sub.2 = 40.7                            r.sub.4 = 0.2803                                                                        d.sub.4 = 0.0456                                                    r.sub.5 = 0.2950                                                                        d.sub.5 = 0.0864                                                                          n.sub.3 = 1.67003                                                                        ν.sub.3 = 47.3                            r.sub.6 = 0.3038                                                                        d.sub.6 = 0.1816                                                    r.sub.7 = 0.3076                                                                        d.sub.7 = 0.0194                                                                          n.sub.4 = 1.71736                                                                        ν.sub.4 = 29.5                            r.sub.8 = 2.4272                                                                        d.sub.8 = 0.1214                                                                          n.sub.5 = 1.69680                                                                        ν.sub.5 = 55.5                            r.sub.9 = d.sub.9 = -0.0024                                                   r.sub.10 = -24.2718                                                                     d.sub.10 = 0.0631                                                                         n.sub.6 = 1.83400                                                                        ν.sub.6 = 37.2                            r.sub.11 = -0.8295                                                            ______________________________________                                         f.sub.2 = -1.725                                                         

This is another embodiment of FIG. 1(A). Each of the symbols used hereinis the same as that used in Embodiment 1.

    ______________________________________                                        Seidel's aberrations of Embodiment 3.                                         S         C         A         P       D                                       ______________________________________                                        1   0.3048    0.1765    0.1022  0.4489  0.3191                                2   0.0224    -0.0905   0.3650  -0.1234 -0.9751                               3   0.6404    0.1760    0.0484  0.8926  0.2586                                4   -6.9706   -0.4731   -0.0321 -1.3115 -0.0912                               5   6.1149    0.8681    0.1232  1.3601  0.2106                                6   -1.7847   -0.7033   -0.2772 -1.3206 -0.6297                               7   -4.5250   1.5041    -0.5000 -1.3580 0.6176                                8   -0.0387   -0.0334   -0.0288 -0.0029 -0.0274                               9   3.3309    0.6375    0.1220  1.0806  -0.2302                               10  -0.0048   0.0257    -0.1366 -0.0187 0.8253                                11  3.3502    -0.8033   0.1926  0.5482  -0.1776                               ______________________________________                                        Σ                                                                           0.4400    0.0092    -0.0213 0.1954  0.1000                                ______________________________________                                    

Seidel's aberrations show the value when the distance of the object sideis set to an infinite point and a diaphragm is set to a position of0.091 rearward of the sixth lens surface, and the symbols in the tableare as follows.

S: spherical aberration

C: coma

A: astigmatism

P: Petzval's condition

D: distortion

Σ: sum of each of the aberrations

EMBODIMENT 4

    ______________________________________                                        f = 1 F 1.84                                                                            2ω = 56°                                                                     f.sub.B = 0.8434                                                                         Σd = 0.6775                            ______________________________________                                        r.sub.1 = 0.9841                                                                        d.sub.1 = 0.0700                                                                          n.sub.1 = 1.71300                                                                        ν.sub.1 = 53.9                            r.sub.2 = 3.8830                                                                        d.sub.2 = 0.0024                                                    r.sub.3 = 0.4285                                                                        d.sub.3 = 0.0459                                                                          n.sub.2 = 1.57845                                                                        ν.sub.2 = 41.5                            r.sub.4 = 0.2865                                                                        d.sub.4 = 0.0483                                                    r.sub.5 = 0.3048                                                                        d.sub.5 = 0.0954                                                                          n.sub.3 = 1.67003                                                                        ν.sub.3 = 47.3                            r.sub.6 = 0.3198                                                                        d.sub.6 = 0.1981                                                    r.sub.7 = -0.3290                                                                       d.sub.7 = 0.0205                                                                          n.sub.4 = 1.76180                                                                        ν.sub.4 = 27.1                            r.sub.8 = 11.5942                                                                       d.sub.8 = 0.0024                                                    r.sub.9 = 24.1546                                                                       d.sub.9 = 0.1292                                                                          n.sub.5 =  1.71300                                                                       ν.sub.5 = 53.9                            r.sub.10 = -0.3871                                                                      d.sub.10 = 0.0024                                                   r.sub.11 = 5.6763                                                                       d.sub.11 = 0.0628                                                                         n.sub.6 = 1.83400                                                                        ν.sub.6 = 37.2                            r.sub.12 = -1.0360                                                            ______________________________________                                         f.sub.2 = -1.695                                                         

This is an embodiment of FIG. 1(B), and where,

r₁,r₂, . . . r₁₂ designates the radius of curvature of the lensesmeasured in sequence from the object side,

d₁,d₃,d₅,d₇,d₉,d₁₁ designates the axial thickness of the lenses measuredin sequence from the object side,

d₂,d₄,d₆,d₈,d₁₀ designates the axial air spaces between the lensesmeasured in sequence from the object side,

n₁,n₂, . . . n₆ designates the refractive index of the optical glass ofthe lenses with respect to the d-line measured in sequence from theobject side, and

ν₁,ν₂, . . . ν₆ designates the Abbe number of the optical glass of thelenses measured with respect to d-line in sequence from the object side.

    ______________________________________                                        Seidel's aberrations of Embodiment 4.                                         S         C         A         P       D                                       ______________________________________                                        1   0.2549    0.1553    0.0946  0.4229  0.3152                                2   0.0255    -0.0972   0.3705  -0.1072 -1.0030                               3   0.5645    0.1531    0.0415  0.8552  0.2433                                4   -6.8606   -0.3101   -0.0140 -1.2792 -0.0585                               5   5.8792    0.7359    0.0921  1.3162  0.1763                                6   -1.4179   -0.5926   -0.2476 -1.2544 -0.6277                               7   -3.7501   1.3339    -0.4744 -1.3143 0.6362                                8   -1.5504   -1.7222   -1.9131 -0.0373 -2.1667                               9   1.4343    1.6240    1.8387  0.0172  2.1013                                10  3.2868    -0.5480   0.0914  1.0751  -0.1945                               11  -0.0002   0.0044    -0.1142 0.0801  0.8763                                12  2.5109    -0.7102   0.2009  0.4389  -0.1810                               ______________________________________                                        Σ                                                                           0.3769    0.0263    -0.0338 0.2132  0.1173                                ______________________________________                                    

Seidel's aberrations show the value when the distance of the object sideis set to an infinite point and a diaphragm is set to a position of0.099 rearward of the sixth lens surface, and the symbols in the tableare as follows.

S: spherical aberration

C: coma

A: astigmatism

P: Petzval's condition

D: distortion

Σ: sum of each of the aberrations

EMBODIMENT 5

    ______________________________________                                        f = 1 F 2 2ω = 56°                                                                     f.sub.B = 0.8538                                                                         Σd = 0.6389                            ______________________________________                                        r.sub.1 = 0.9814                                                                        d.sub.1 = 0.0685                                                                          n.sub.1 = 1.71300                                                                        ν.sub.1 = 53.9                            r.sub.2 = 3.7090                                                                        d.sub.2 = 0.0024                                                    r.sub.3 = 0.4053                                                                        d.sub.3 = 0.0434                                                                          n.sub.2 = 1.58144                                                                        ν.sub.2 = 40.7                            r.sub.4 = 0.2796                                                                        d.sub.4 = 0.0454                                                    r.sub.5 = 0.2961                                                                        d.sub.5 = 0.0869                                                                          n.sub.3 = 1.67003                                                                        ν.sub.3 = 47.3                            r.sub.6 = 0.3076                                                                        d.sub.6 = 0.1834                                                    r.sub.7 = -0.3132                                                                       d.sub.7 = 0.0193                                                                          n.sub.4 = 1.72825                                                                        ν.sub.4 = 28.5                            r.sub.8 = 4.0871                                                                        d.sub.8 = 0.0014                                                    r.sub.9 = 4.4515                                                                        d.sub.9 = 0.1219                                                                          n.sub.5 = 1.69680                                                                        ν.sub.5 = 55.5                            r.sub.10 =  -0.3747                                                                     d.sub.10 = 0.0024                                                   r.sub.11 = 16.6489                                                                      d.sub.11 = 0.0639                                                                         n.sub.6 = 1.83400                                                                        ν.sub.6 = 37.2                            r.sub.12 = -0.9391                                                            ______________________________________                                         f.sub.2 = -1.776                                                         

This is another embodiment of FIG. 1(B). Each of the symbols used hereinis the same as that used in Embodiment 4.

    ______________________________________                                        Seidel's aberrations of Embodiment 5.                                         S         C         A         P       D                                       ______________________________________                                        1   0.2560    0.1630    0.1038  0.4241  0.3363                                2   0.0219    -0.0878   0.3526  -0.1122 -0.9655                               3   0.7694    0.1963    0.0501  0.9072  0.2443                                4   -7.2221   -0.4641   -0.0298 -1.3151 -0.0864                               5   6.2365    0.8686    0.1210  1.3552  0.2056                                6   -1.7196   -0.6840   -0.2721 -1.3041 -0.6270                               7   -4.4021   1.4818    -0.4988 -1.3454 0.6208                                8   -2.3472   -2.2302   -2.1190 -0.1031 -2.1113                               9   2.2635    2.1663    2.0734  0.0923  2.0727                                10  3.6913    -0.6397   0.1109  1.0960  -0.2092                               11  -0.0023   0.0178    -0.1362 0.0273  0.8302                                12  2.8765    -0.7851   0.2143  0.4842  -0.1906                               ______________________________________                                        Σ                                                                           0.4217    0.0032    -0.0299 0.2063  0.1198                                ______________________________________                                    

Seidel's aberrations show the value when the distance of the object sideis set to an infinite point and a diaphragm is set to a position of0.092 rearward of the sixth lens surface, and the symbols in the tableare as follows.

S: spherical aberration

C: coma

A: astigmatism

P: Petzval's condition

D: distortion

Σ: sum of each of the aberrations

What is claimed is:
 1. A semi wide angle objective lens constituting offive groups of six lens elements formed a front group having the firstgroup of a positive meniscus single lens L₁ with its convex surfacedirected toward the object side, the second group of a negative meniscussingle lens L₂ with its convex surface directed toward the object sideand the third group of a positive meniscus single lens L₃ with itsconvex surface directed toward the object side, and forming a rear grouphaving the fourth group of a negative lens L₄ with its concave surfacehaving a large curvature directed toward the object side and of apositive lens L₅ with its convex surface having a large curvaturedirected toward an image side which are arranged with a mutual contactcondition therebetween, and the fifth group of a positive lens L₆characterized in that each of the following conditions, i.e.

    1.69 f<|f.sub.2 |<1.79f                  (1)

    0.27 f<r.sub.4 <0.29 f                                     (2)

    0.045 f<d.sub.4 <0.054 f                                   (3)

    0.94<r.sub.4 /r.sub.5 <0.096                               (4)

where, f: the composite focal length of the entire lens system f₂ : thesingle focal length of the lens L₂ of the second group r₄ : the radiusof curvature of the rear surface of the lens L₂ of the second group d₄ :the axial air space between the lens L₂ of the second group and lens L₃of the third group r₅ : the radius of curvature of the front surface ofthe lens L₃ of the third groupis fulfilled.
 2. Semi wide angle objectivelens for fulfilling the condition as set forth in claim 1 constructedaccording to the constructional data given herebelow:

    ______________________________________                                        f = 1 F 1.8                                                                             2ω = 56°                                                                     f.sub.B = 0.8464                                        ______________________________________                                        r.sub.1 = 0.9951                                                                        d.sub.1 = 0.0725                                                                          n.sub.1 = 1.71300                                                                        ν.sub.1 = 53.9                            r.sub.2 = 3.9769                                                                        d.sub.2 = 0.0024                                                    r.sub.3 = 0.4274                                                                        d.sub.3 = 0.0483                                                                          n.sub.2 = 1.58144                                                                        ν.sub.2 = 40.7                            r.sub.4 = 0.2873                                                                        d.sub.4 = 0.0531                                                    r.sub.5 = 0.3052                                                                        d.sub.5 = 0.0966                                                                          n.sub.3 = 1.67003                                                                        ν.sub.3 = 47.3                            r.sub.6 = 0.3126                                                                        d.sub.6 = 0.2029                                                    r.sub.7 = -0.3341                                                                       d.sub.7 = 0.0217                                                                          n.sub.4 = 1.75520                                                                        ν.sub.4 = 27.5                            r.sub.8 = 38.9089                                                                       d.sub.8 = 0.1329                                                                          n.sub.5 = 1.67790                                                                        ν.sub.5 = 55.3                            r.sub.9 = -0.3966                                                                       d.sub.9  = 0.0024                                                   r.sub.10 = 11.2319                                                                      d.sub.10 = 0.0725                                                                         n.sub.6 = 1.88300                                                                        ν.sub.6 = 40.8                            r.sub.11 = -0.9167                                                            ______________________________________                                    

where, r₁,r₂, . . . r₁₁ : the radius of curvature of the lens measuredin sequence from the object side d₁,d₃,d₅,d₇,d₈,d₁₀ : the axialthickness of the lens measured in sequence from the object sided₂,d₄,d₆,d₉ : the axial air space between the lenses measured insequence from the object side n₁,n₂, . . . n₆ : the refractive index ofthe optical glass of the lens with respect to the d-line measured insequence from the object side ν₁,ν₂, . . . ν₆ : Abbe number of theoptical glass of the lens measured with respect to d-line in sequencefrom the object side.
 3. Semi wide angle objective lens for fulfillingthe condition as set forth in claim 1 constructed according to theconstructional data given herebelow:

    ______________________________________                                        f = 1 F 1.84                                                                            2ω = 56°                                                                     f.sub.B = 0.8443                                        ______________________________________                                        r.sub.1 = 0.9963                                                                        d.sub.1 = 0.0724                                                                          n.sub.1 = 1.71300                                                                        ν.sub.1 = 53.9                            r.sub.2 = 3.9165                                                                        d.sub.2 = 0.0024                                                    r.sub.3 = 0.4165                                                                        d.sub.3 = 0.0458                                                                          n.sub.2 = 1.58144                                                                        ν.sub.2 = 40.7                            r.sub.4 = 0.2851                                                                        d.sub.4 = 0.0531                                                    r.sub.5 = 0.3015                                                                        d.sub.5 = 0.0965                                                                          n.sub.3 = 1.67003                                                                        ν.sub.3 = 47.3                            r.sub.6 = 0.3057                                                                        d.sub.6 = 0.1978                                                    r.sub.7 = -0.3287                                                                       d.sub.7 = 0.0210                                                                          n.sub.4 = 1.76182                                                                        ν.sub.4 = 26.6                            r.sub.8 = 8.4698                                                                        d.sub.8 = 0.1303                                                                          n.sub.5 = 1.71300                                                                        ν.sub.5 = 53.9                            r.sub.9 = -0.3998                                                                       d.sub.9  = 0.0024                                                   r.sub.10 = 11.3076                                                                      d.sub.10 = 0.0724                                                                         n.sub.6 = 1.83400                                                                        ν.sub.6 = 37.2                            r.sub.11 = -0.8810                                                            ______________________________________                                    

where, r₁,r₂, . . . r₁₁ : the radius of curvature of the lens measuredin sequence from the object side d₁,d₃,d₅,d₇,d₈,d₁₀ : the axialthickness of the lens measured in sequence from the object sided₂,d₄,d₆,d₉ : the axial air space between the lenses measured insequence from the object side n₁,n₂, . . . n₆ : the refractive index ofthe optical glass of the lens with respect to the d-line measured insequence from the object side ν₁,ν₂, . . . ν₆ : Abbe number of theoptical glass of a lens measured with respect to d-line in sequence fromthe object side.
 4. Semi wide angle objective lens for fulfilling thecondition as set forth in claim 1 constructed according to theconstructional data given herebelow:

    ______________________________________                                        f = 1 F 2 2ω = 56°                                                                     f.sub.B = 0.8518                                        ______________________________________                                        r.sub.1 = 0.9272                                                                        d.sub.1 = 0.0689                                                                          n.sub.1 = 1.71300                                                                        ν.sub.1 = 53.9                            r.sub.2 = 3.3721                                                                        d.sub.2 = 0.0024                                                    r.sub.3 = 0.4119                                                                        d.sub.3 = 0.0447                                                                          n.sub.2 = 1.58144                                                                        ν.sub.2 = 40.7                            r.sub.4 = 0.2803                                                                        d.sub.4 = 0.0456                                                    r.sub.5 = 0.2950                                                                        d.sub.5 = 0.0864                                                                          n.sub.3 = 1.67003                                                                        ν.sub.3 = 47.3                            r.sub.6 = 0.3038                                                                        d.sub.6 = 0.1816                                                    r.sub.7 = -0.3076                                                                       d.sub.7 = 0.0194                                                                          n.sub.4 = 1.71736                                                                        ν.sub.4 = 29.5                            r.sub.8 = 2.4272                                                                        d.sub.8 = 0.1214                                                                          n.sub.5 = 1.69680                                                                        ν.sub.5 = 55.5                            r.sub.9 = -0.3800                                                                       d.sub.9  = 0.0024                                                   r.sub.10 = -24.2718                                                                     d.sub.10 = 0.0631                                                                         n.sub.6 = 1.83400                                                                        ν.sub.6 = 37.2                            r.sub.11 = -0.8295                                                            ______________________________________                                    

where, r₁,r₂, . . . r₁₁ : the radius of curvature of the lens measuredin sequence from the object side d₁,d₃,d₅,d₇,d₈,d₁₀ : the axialthickness of the lens measured in sequence from the object sided₂,d₄,d₆,d₉ : the axial air space between the lenses measured insequence from the object side n₁,n₂, . . . n₆ : the refractive index ofthe optical glass of the lens with respect to the d-line measured insequence from the object side ν₁,ν₂, . . . ν₆ : Abbe number of theoptical glass of the lens measured with respect to d-line in sequencefrom the object side.
 5. Semi wide angle objective lens for fulfillingthe condition as set forth in claim 1 constructed according to theconstructional data given herebelow:

    ______________________________________                                        f = 1 F 1.84                                                                            2ω = 56°                                                                     f.sub.B = 0.8434                                        ______________________________________                                        r.sub.1 = 0.9841                                                                        d.sub.1 = 0.0700                                                                          n.sub.1 = 1.71300                                                                        ν.sub.1 = 53.9                            r.sub.2 = 3.8830                                                                        d.sub.2 = 0.0024                                                    r.sub.3 = 0.4285                                                                        d.sub.3 = 0.0459                                                                          n.sub.2 = 1.57845                                                                        ν.sub.2 = 41.5                            r.sub.4 = 0.2865                                                                        d.sub.4 = 0.0483                                                    r.sub.5 = 0.3048                                                                        d.sub.5 = 0.0954                                                                          n.sub.3 = 1.67003                                                                        ν.sub.3 = 47.3                            r.sub.6 = 0.3198                                                                        d.sub.6 = 0.1981                                                    r.sub.7 = -0.3290                                                                       d.sub.7 = 0.0205                                                                          n.sub.4 = 1.76180                                                                        ν.sub.4 = 27.1                            r.sub.8 = 11.5942                                                                       d.sub.8 = 0.0024                                                    r.sub.9 = 24.1546                                                                       d.sub.9 = 0.1292                                                                          n.sub.5 = 1.71300                                                                        ν.sub.5 =  53.9                           r.sub.10 = -0.3871                                                                      d.sub.10 = 0.0024                                                   r.sub.11 = 5.6763                                                                       d.sub.11 = 0.0628                                                                         n.sub.6 = 1.83400                                                                        ν.sub.6 = 37.2                            r.sub.12 = -1.0360                                                            ______________________________________                                    

where, r₁,r₂, . . . r₁₂ : the radius of curvature of the lens measuredin sequence from the object side d₁,d₃,d₅,d₇,d₉,d₁₁ : the axialthickness of the lens measured in sequence from the object sided₂,d₄,d₆,d₈,d₁₀ : the axial air space between the lenses measured insequence from the object side n₁,n₂, . . . n₆ : the refractive index ofthe optical glass of the lens with respect to the d-line measured insequence from the object side ν₁,ν₂, . . . ν₆ : Abbe number of theoptical glass of the lens measured with respect to d-line in sequencefrom the object side.
 6. Semi wide angle objective lens for fulfillingthe condition as set forth in claim 1 constructed according to theconstructional data given herebelow:

    ______________________________________                                        f =0 1 F 2                                                                              2ω = 56°                                                                     f.sub.B = 0.8538                                        ______________________________________                                        r.sub.1 = 0.9814                                                                        d.sub.1 = 0.0685                                                                          n.sub.1 = 1.71300                                                                        ν.sub.1 = 53.9                            r.sub.2 = 3.7090                                                                        d.sub.2 = 0.0024                                                    r.sub.3 = 0.4053                                                                        d.sub.3 = 0.0434                                                                          n.sub.2 = 1.58144                                                                        ν.sub.2 = 40.7                            r.sub.4 = 0.2796                                                                        d.sub.4 = 0.0454                                                    r.sub.5 = 0.2961                                                                        d.sub.5 = 0.0869                                                                          n.sub.3 = 1.67003                                                                        ν.sub.3 = 47.3                            r.sub.6 = 0.3076                                                                        d.sub.6 = 0.1834                                                    r.sub.7 = -0.3132                                                                       d.sub.7 = 0.0193                                                                          n.sub.4 = 1.72825                                                                        ν.sub.4 = 28.5                            r.sub.8 = 4.0871                                                                        d.sub.8 = 0.0014                                                    r.sub.9 = 4.4515                                                                        d.sub.9 = 0.1219                                                                          n.sub.5 = 1.69680                                                                        ν.sub.5 =  55.5                           r.sub.10 = -0.3747                                                                      d.sub.10 = 0.0024                                                   r.sub.11 = 16.6489                                                                      d.sub.11 = 0.0639                                                                         n.sub.6 = 1.83400                                                                        ν.sub.6 = 37.2                            r.sub.12 = -0.9391                                                            ______________________________________                                    

where, r₁,r₂, . . . r₁₂ : the radius of curvature of the lens measuredin sequence from the object side d₁,d₃,d₅,d₇,d₉,d₁₁ : the axialthickness of the lens measured in sequence from the object sided₂,d₄,d₆,d₈,d₁₀ : the axial air space between the lenses measured insequence from the object side n₁,n₂, . . . n₆ : the refractive index ofthe optical glass of the lens with respect to the d-line measured insequence from the object side ν₁. ν₂, . . . ν₆ : Abbe number of theoptical glass of the lens measured with respect to d-line in sequencefrom the object side.
 7. A semi wide angle objective lens constitutingof five groups of six lens elements forming a front group having thefirst group of a positive meniscus single lens L₁ with its convexsurface directed toward the object side, the second group of a negativemeniscus single lens L₂ with its convex surface directed toward theobject side and the third group of a positive meniscus single lens L₃with its convex surface directed toward the object side, and forming arear group having the fourth group of a negative lens L₄ with itsconcave surface having a large curvature directed toward the object sideand of a positive lens L₅ with its convex surface having a largecurvature directed toward an image side which are arranged with anextremely small air space therebetween, and the fifth group of apositive lens L₆ characterized in that each of the following conditions,i.e.

    1.69 f<|f.sub.2 |<1.79 f                 (1)

    0.27 f<r.sub.4 <0.29 f                                     (2)

    0.045 f<d.sub.4 <0.054 f                                   (3)

    0.94<r.sub.4 /r.sub.5 <0.96                                (4)

where f: the composite focal length of the entire lens system f₂ : thesingle focal length of the lens L₂ of the second group r₄ : the radiusof curvature of the rear surface of the lens L₂ of the second group d₄ :the axial air space between the L₂ of the second group and lens L₃ ofthe third group r₅ : the radius of curvature of the front surface of thelens L₃ of the third groupis fulfilled.